Fixed the installer when running it with PHP 5.2
Fixed the installer when running it with PHP 5.2

One of the checks performed by this installer is that PHP 5.3+ is used, so it should not used the FQCN to access the RuntimeException class so that the code can be parsed by PHP 5.2
composer/getcomposer.org,johnstevenson/getcomposer.org,johnstevenson/getcomposer.org,johnstevenson/getcomposer.org,composer/getcomposer.org,composer/getcomposer.org